This grant application proposes the design, synthesis, and evaluation (in vitro and in vivo) of new PET (positron Emission Tomography) radioligands for in vivo imaging of the ?---aminobutyric acid (GABA) transporter localized in the presynaptic neuronal membrane. Although GABA is one of the predominant inhibitory neurotransmitters in the mammalian brain, and dysfunctioning of the GABA system is evident in such important human diseases as epilepsy, schizophrenia and autism, there are currently no in vivo imaging agents for studies of the presynaptic GABA---ergic innervation in the human brain. The specific aims of this project are the synthesis of a series of diarylalkenyl ether nipecotic acid analogs of the clinically used drug tiagabine (Gabatril(R)) and evaluation of their in vitro binding affinitie and lipophilicity. Candidate compounds with low nanomolar affinities, and appropriate lipophilicity, will be radiolabeled with carbon---11 or fluorine---18 and tested in vivo in rodentsto determine blood---brain---barrier permeability, metabolism, brain pharmacokinetics, and pharmacological specificity for the GABA transporter type 1 (GAT---1). The most promising radiotracers will then be evaluated in the rhesus monkey brain, to determine pharmacokinetics and potential for quantitative measures of GABA---ergic neuron innervation. The goal of the project is to identify candidate radioligands with suitable brain uptake and pharmacokinetics for further development into PET radiopharmaceuticals for non---invasive studies of the GABA neuronal system in human neurological and psychiatric diseases.